1. Field of the Invention
This invention relates generally to electrically heated switch devices and more particularly, it relates to a self regulating heated switch assembly. More specifically, the invention is directed to a self regulating heated switch assembly which utilizes a positive temperature coefficient (PTC) resistive material adapted for heating the interior of a switch housing so as to prevent freezing of switch contacts therein.
2. Description of the Prior Art
It is generally well known in the art that switches which are subjected to low ambient temperatures, such as below freezing, encounter condensation of moisture so as to often cause electrical shorts thereby rendering the switches inoperative. Moreover, when such condensed moisture freezes on the switch contacts at the low temperature environments the switch closing operation providing electrical connection fails to occur. Therefore, it would be desirable to provide a switch assembly which can be maintained at above freezing temperatures, even when utilized in below freezing environments so as to reduce the condensed moisture and thus prevent freezing thereof on the switch contacts.
A prior art search directed to the subject matter of this invention in the U.S. Patent and Trademark Office revealed the following U.S. Letters Patent:
______________________________________ 2,632,083 4,318,070 4,808,960 3,179,544 4,352,008 4,823,104 3,192,345 4,399,423 4,849,729 3,402,280 4,631,391 4,857,711 3,611,235 4,689,595 4,862,309 3,865,626 4,703,298 4,878,038 4,174,511 4,743,321 Re 31,367 ______________________________________
In U.S. Pat. No. 2,632,083 to Stephen R. Shaffer issued on Mar. 17, 1953, there is disclosed a switch housing unit which includes a resistance heating coil mounted between a panel and a bottom wall of the switch housing. A temperature responsive switch (thermostat) is provided for selectively energizing the heating coil so as to electrically heat the switch unit. In U.S. Pat. No. 4,352,008 to Edwin Hofer et al., issued on Sep. 28, 1982, there is disclosed an electric heating device for heating the interior of a switch cabinet so as to prevent condensation therein which includes a PTC electrical resistance heating and temperature regulating device which is situated in the interior of a body made of a thermally conductive material that is thermally connected to a radiator. The radiator is formed by a pair of flat base plates extending laterally from opposite sides of the body and radiator fins extending only from the base plates and only from one flat side thereof.
U.S. Pat. No. 4,631,391 to Gunter Piepke issued on Dec. 23, 1986, teaches an electrical heating device for a mirror which includes a pair of electrically conductive heating lining plates to provide heat distribution on the mirror and electrical conductors 9 connected to the plates for supplying a source of power. An insulating layer 3 is provided in two places with spaced openings 13 into which are mounted PTC platelettes 4 that are in electrical contact with the pair of plates. The heating device is adapted to be adhered to the backside of a mirror glass for defogging and demisting the mirror.
U.S. Pat. No. 4,857,711 to Leslie M. Walts issued on Aug. 15, 1989, and assigned to the same assignee as in the present invention teaches a self regulating heating device for a mirror which includes a substrate 14 having an electrical buss system deposited on one surface and being formed of a plurality of interdigitated electrodes 32, 34, 36, 38 and two buss bars 16, 18. Stripes of positive temperature coefficient (PTC) resistive material are printed perpendicularly over the buss system to form a plurality of heating areas and exposed substrate areas. The self regulating heating device is adapted for use in heating automotive-type outside rearview mirrors.
Numerous other switch devices utilizing PTC elements were uncovered in the prior art search in which the PTC element was typically employed to serve as a circuit protection or cut-off device such as by heating a bi-metal contact, as evidenced by U.S. Pat. Nos. 4,823,104; 4,849,729; 4,878,038 and Re. 31,367. The remaining patents listed, but not specifically discussed, are deemed to be of only general interest and are cited to show the state of the art in positive temperature coefficient (PTC) materials and heated switch devices.
However, none of the prior art uncovered in the search disclosed a self regulating heated switch assembly like that of the present invention which includes a switch housing, an insulative cover member, and a PTC heater device sandwiched between the switch housing and the cover member so as to transmit heat into the interior of the switch housing. The switch assembly can be maintained at above freezing temperatures, even when utilized in below freezing environments, so as to reduce the condensed moisture and thus prevent freezing thereof on switch contacts that are in the switch housing.